The present invention relates to a construction for closing a through hole in a Mg alloy case.
Conventionally, an oil pan for an engine as a case is formed of Al alloy or steel, and a drain hole, which is a through hole in the oil pan which is provided with internal threads, is closed with a screw type steel plug.
The idea of constructing oil pans of Mg alloy has resulted from increasing demands for lighter oil pans. However, if the aforesaid closing means is adopted for a Mg oil pan, since the Mg alloy is located on the lowest potential side of a corrosion potential range and rain water tends to easily collect at a peripheral portion of an opening of the drain hole while running, the peripheral portion of the opening becomes anode from a relationship with the material of the plug and gets corroded.
With a view to reducing further the weight of the case, an object of the invention is to construct the case of Mg alloy and to provide a construction for closing a through hole in the Mg alloy case which can securely avoid corrosion at the peripheral portion of the opening of the through hole in the case.
With a view to attaining the above object, according to the invention, there is provided a construction for closing a through hole in a Mg alloy case in which the through hole in the Mg alloy case which is provided with internal threads is closed with a screw type plug body formed of a metal other than Mg and Mg alloy, the construction being characterized in that the plug body has a threaded shank screwed into the through hole and an enlarged end having a diameter greater than that of the threaded shank, and that a corrosion protecting plate fitted on the threaded shank via a threaded shank passing hole is disposed between the enlarged end and a peripheral portion of an opening of the through hole which corresponds to the enlarged end so that the enlarged end and the peripheral portion of the opening are electrically insulated, the corrosion protecting plate having a plurality of annular sealing projections on both sides thereof which are concentrically disposed about the threaded shank passing hole so as to bite into the peripheral portion of the opening and the enlarged end.
As is described above, when the enlarged end of the plug body and the peripheral portion of the opening of the through hole are electrically insulated from each other with the corrosion protecting plate, even if rain water collects around the corrosion protecting plate, since the peripheral portion of the opening is prevented from becoming anode, the peripheral portion of the opening can be prevented from corrosion.
Additionally, since there is provided secure sealing between the peripheral portion of the opening and the corrosion protecting plate, as well as between the enlarged end and the corrosion protecting plate through the biting engagement of the plurality of annular sealing projections, the intrusion of water into the through hole is prevented, whereby the corrosion of the internal threads of the through hole can be avoided. Although it is natural to happen, the sealing construction can securely prevents the leakage of the contents of the case to the outside thereof.